A known torque fluctuation absorber is arranged on a power transmission path between an engine and a transmission, and absorbs (decreases or restricts) torque fluctuations generated between the engine and the transmission. The torque fluctuation absorber may include a hysteresis mechanism for absorbing the torque fluctuations by means of a hysteresis torque generated by a frictional force. At the hysteresis mechanism, a thrust member arranged between two rotating members in an axial direction is pressed against one of the rotating members by means of a disc spring. In a case where a torsion is generated between the mentioned two rotating members, the torque fluctuations are absorbed by means of the hysteresis torque generated by the frictional force between the thrust member and the mentioned one of the rotating members. Such hysteresis mechanism may also include a mechanism (variable hysteresis mechanism) which changes the hysteresis torque in accordance with a torsion angle between two rotating members.
A known clutch disc including the variable hysteresis mechanism is disclosed in JPS58-637A (hereinafter referred to as Patent reference 1). According to the construction disclosed in Patent reference 1, in a case where a torsion is generated between conducting plate members and a clutch hub, the variable hysteresis mechanism shifts a control member in an axial direction. The control member has a cam face which makes contact with and engages with a contact-sliding member formed at one of the conducting plate members. Then, a load of an elastic member arranged between a flange portion of the clutch hub and the control member is changed, and accordingly, a frictional force of a friction member arranged between the flange portion of the clutch hub and the conducting plate member is changed. Thus, the variable hysteresis mechanism changes a hysteresis torque.
Another known clutch disc including the variable hysteresis mechanism is disclosed in JPH1-158225A (hereinafter referred to as Patent reference 2). In a case where a torsion is generated between disc members and a hub member, according to the construction disclosed in Patent reference 2, the variable hysteresis mechanism changes a load of a spring member having protruding portions which slidably make contact with a cam face formed at a plate member, and accordingly, a frictional force of a friction member pressed against a hub member by a spring member is changed. Thus, the variable hysteresis mechanism changes a hysteresis torque.
A known torque fluctuation absorber is disclosed in JP2007-218347A (hereinafter referred to as Patent reference 3). The torque fluctuation absorber, according to Patent reference 3, includes a first hysteresis mechanism configuring with side plates and a hub and generating a smaller hysteresis by biasing low friction members towards the side plates or towards the hub within a rotation allowance space where the side plates and the hub are allowed to rotate, and a second hysteresis mechanism generating a larger hysteresis by biasing high friction members towards the side plates or the hub out of the movement and rotation allowance space.
Another known clutch disc including the variable hysteresis mechanism is disclosed in WO2011060752 pamphlet (hereinafter referred to as Patent reference 4). In a case where a torsion is generated between disc members and an inertial body, according to the construction of Patent reference 4, the variable hysteresis mechanism shifts one of two inclined surface members frictionally engaged with one another in an axial direction. Then, a load applied to a spring arranged between one of the inclined surface members and the disc member is changed, and accordingly, a frictional force generated between the mentioned two inclined surface members is changed. Thus, the variable hysteresis mechanism changes a hysteresis torque.
According to the variable hysteresis mechanism described in the Patent reference 1, the frictional force generated between the contact sliding member and the cam face, serving as a cam mechanism, rarely changes, and substantially, only the frictional force of the friction member arranged between the flange portion of the clutch hub and the conducting plate member changes. Thus, it may be difficult to generate a large hysteresis torque.
According to the variable hysteresis mechanism described in the Patent reference 2, it may be difficult to maintain a strength of the variable hysteresis mechanism against an excessive torque because a cam mechanism is arranged at the spring member which has a bending characteristic.
According to the variable hysteresis mechanism described in the Patent reference 3, the variable hysteresis mechanism is configured such that the hysteresis torque changes in accordance with a change in torque fluctuations that are absorbed within the rotation allowance space, or out of the rotation allowance space. Thus, it is difficult to change the hysteresis torque smoothly.
According to the variable hysteresis mechanism described in the Patent reference 4, the hysteresis torque generated by a torsion angle between the disc member and the inertial body is symmetric (symmetric with respect to point, see FIG. 11) irrespective of a torsion direction. Thus, the variable hysteresis mechanism is not suitable for a device in which a torsion characteristic should be changed in accordance with the torsion direction and the torsion angle.
A need thus exists for a torque fluctuation absorber which is not susceptible to the drawback mentioned above.